This invention relates to orthotics, to supports or stabilizers for joints and, in particular, to a knee brace which serves both preventive and remedial functions in protecting against medial-lateral, anterior-posterior and rotatory instabilities.
The knee joint is perhaps the most susceptible to injury of the major articulated joints of the human body, despite the presence of five major ligaments and two menisci which serve to connect and stabilize the tibia and femur. These structures include the anterior and posterior cruciate ligaments, the medial and lateral collateral ligaments, the posterior capsule ligament and the medial and lateral menisci.
Anatomically, the knee is designed so that specific muscles or muscle groups, not ligaments, absorb the brunt of external or internal forces. That is, a muscle or group of muscles substitutes for each ligament in the knee to absorb force and restrict motion. As examples, the hamstrings substitute for the anterior cruciate ligament, the quadriceps for the posterior cruciate ligament, and the abductor and adductor groups for the medial and lateral collateral ligaments.
The articulation of the knee joint, and the ligaments, muscles and bones associated with the joint are described, for example, in Gray's Anatomy and in The Johns Hopkins Atlas of Human Functional Anatomy, 2d ed., 1980. These teachings are incorporated by reference.
When a muscle is unable to completely absorb an applied force, either because of inherent weakness or prior injury or simply because the force is too strong, the unabsorbed component of force is transmitted to one or more ligaments. If the transmitted component is sufficiently great, the ligament is strained or torn. Ligamental susceptibility to injury is also dependent upon the degree of flexion or extension. Typically, if the femur and tibia are in a relatively straight orientation or if the knee is slightly extended, the ligaments are fairly tight. This reduces the amount of displacement of the tibia and femur and the chance of injury. However, the inherent cooperation and relationship among the ligaments is such that when the knee is bent or flexed, some ligaments are relatively tight and tend to control displacement, but others are relatively loose. Between 20.degree.-60.degree. of flexion the knee is very susceptible to displacement and to injury. This is unfortunate, because the knee is frequently in this position, particularly during the more active sports activities. It is factors such as these which make the knee relatively weak compared to the other major articulated joints. Some, such as the ball and socket hip joint, are very secure. Others, such as the arm and shoulder joints are complicated but nonetheless relatively secure. Despite its inherent weaknesses, the knee joint must both support the weight of the body and provide for movement, while holding the tibia and femur in position along their substantially planar unstable interface. In considering external forces applied to the knee and the resulting ligament injuries, it is helpful to simplify the situation somewhat and consider the forces as having their major components applied primarily along a frontal plane through the knee, or along a sagittal plane through the knee, or as comprising a rotatory force. Frontal plane forces are medial-lateral forces which displace the femur and/or tibia in a side-to-side direction. Saggital plane forces are anterior-posterior forces which displace the femur and tibia in approximately a front-to-back motion, and includes drawering forces applied during flexion or extension. Rotatory forces are those which tend to induce relative rotational displacement of or between the femur and tibia, primarily against the stabilizing force provided by the anterior cruciate ligament.
FIGS. 1 through 3 illustrate examples of the above forces. In these schematic drawings, the femur, tibia and knee are respectively designated 11, 12 and 13. Referring specifically to FIG. 1, two of the more common knee ligament injuries result from medial and lateral forces. The medial collateral ligament and lateral collateral ligament are primary stabilizing influences against medial and lateral force, respectively. As a consequence, strains or tears of these ligaments tend to result, respectively, from medial forces, that is, inward or medially-directed forces 14 applied against the outside of the leg, or lateral forces 15, which are outward directed forces applied against the inside of the leg. Perhaps the most frequent injury in sports, and certainly one of the most damaging injuries to the joints involves strains or tears of the anterior cruciate ligament. Referring to the side views shown in FIG. 2, the responsible force may involve an anterior tibial force alone, that is, a forward-directed force 21 applied to the back of the tibia. See FIG. 2A. The force may involve the combination of a rotational tibial force 22 which rotates the tibia relative to the femur (as by catching a shoe or cleats in turf) and an anterior tibial force 21. See FIG. 2B. In either case, injury to the anterior cruciate ligament results from excessive force which the substitutional muscles and the anterior cruciate ligament are unable to absorb and a resulting anterior tibial acceleration and displacement relative to the femur. See FIG. 3. The reason for the frequent occurrence of this injury is two-fold, namely the frequency with which the knee and leg are subjected to large magnitude forces, and the susceptibility to injury in that typically the knee can withstand only about 380 pounds of force and 12.5 millimeters displacement or movement between the tibia and the femur without injury to the anterior cruciate ligament.
Referring to FIG. 4 and as shown by the arrow 41 therein, a posterior-directed force 41 is the opposite of anterior-directed force 21. The knee 13 is stabilized against posterior forces primarily by the posterior cruciate ligament. Unlike the anterior cruciate ligament, the posterior cruciate ligament is backed by the posterior capsule ligament, which is quite effective in stabilizing the knee against displacement. As a result, isolated posterior cruciate tears are rare. Usually injuries to other ligaments are also involved. In fact, it is not infrequent that the bone attachment itself tears rather than, or in addition to the posterior capsule ligament.
Displacement of the femur and tibia resulting from rotational forces such as 22, FIG. 2B, is another primary cause of injury to the anterior cruciate ligament. Of course, if there is existing damage or if the anterior cruciate ligament has inherent instability, the knee is more susceptible to displacement and the ligament is more susceptible to injury. The same is true of the other ligaments in that existing damage or instability increases their susceptibility to injury.
Concentrated efforts by the orthotics' profession to develop knee stabilizers are thought to have been initiated in the 1960's as a result of publicized knee injuries suffered by professional athletes. It is believed basically two types of knee braces have dominated this field. Referring to the FIG. 5 front view, one such brace 50 uses a three-point suspension which is provided by two pads 51 and 52 situated above and below the knee (on either the medial or the lateral side of the leg) and a third pad 53 on the opposite side of the leg adjacent the knee. Rigid braces 54--54 correct the pads. Various straps can be used to enhance suspension and/or stabilization characteristics. Referring to the side view shown in FIG. 6, the second type 60 of conventional knee brace uses relatively rigid anterior femur and tibial shells 61 and 62 which are joined by hinged uprights 63--63 and supported in the back or posterior side by elastic straps 64 and 65. These designs are more effective at protecting against medial-lateral forces than anterior-posterior forces. The reason is simple. The rigid shells/pads and connecting braces provide relatively inflexible pressure points which stabilize against lateral or medial forces. In contrast, the relatively flexible front-to-rear stabilization systems provided by these braces permit relative movement of the tibia and femur along the sagittal plane.
In addition, because rotary stability is a function of both medial-lateral and anterior-posterior stability, the implementation of conventional knee brace designs tends to be less effective than desired in any derotation function. Furthermore, stabilization in all aspects is closely related to the effective suspension of the orthotic device on the knee and leg in a manner such that the device does not alter or shift its position on the leg as by planing. Many prior art devices experience planing and shifting which detract from their ability to provide medial-lateral stability, anterior-posterior stability and/or rotatory stability. In addition to the difficulty of achieving adequate suspension stability using typical prior art knee braces, frequently such braces avoid the problems associated with flexion by restricting movement of the knee. Because of restrictions on movement and because of weight, the use of these braces to prevent injuries puts the athlete at such a competitive disadvantage that knee braces are not widely used for injury prevention. Rather, the primary use has been remedial, to compensate for and protect against existing injuries and weaknesses in already damaged and/or unstable knees. Perhaps the one exception to the use of prior art knee braces for remedial purposes rather than prevention is the class of braces which consist simply of a pair of upright bands on the sides of the leg. These are used to provide some means of protection against medial-lateral forces.